Photon counting device and photon counting method

ABSTRACT

A photon counting device includes a plurality of pixels each including a photoelectric conversion element configured to convert input light to charge, and an amplifier configured to amplify the charge converted by the photoelectric conversion element and convert the charge to a voltage, an A/D converter configured to convert the voltage output from the amplifier of each of the plurality of pixels to a digital value and output the digital value, a correction unit configured to correct the digital value output from the A/D converter so that an influence of a variation in a gain and an offset value among the plurality of pixels is curbed, a calculation unit configured to output a summed value obtained by summing the corrected digital values corresponding to at least two pixels, and a conversion unit configured to convert the summed value output from the calculation unit to a number of photons.

TECHNICAL FIELD

The present disclosure relates to a photon counting device and a photoncounting method.

BACKGROUND ART

For example, Non-Patent Literature 1 describes a photon countingtechnique using a CMOS image sensor. In this technique, imaging isperformed under a condition that only one photon is incident on onepixel in one frame by increasing a frame rate of the image sensor.

CITATION LIST Patent Literature

-   [Non-Patent Literature 1] B Saleh Masoodian, Jiaju Ma, Dakota    Starkey, Yuichiro Yamashita, and Eric R. Fossum, “A 1Mjot 1040 fps    0.22e-rms Stacked BSI Quanta Image Sensor with Cluster-Parallel    Readout”, Proceedings of 2017 International Image Sensor Workshop    (IISW), May 30-Jun. 2, 2017, P230-233

SUMMARY OF INVENTION Technical Problem

For example, when photon counting is performed using a CMOS imagesensor, it is conceivable to discriminate the number of photons on thebasis of a digital value output from an A/D converter. However, in theCMOS image sensor, each of pixels constituting the sensor has readoutnoise. Further, a gain and an offset value of a plurality of the pixelshave a variation in a certain range. In this case, since digital valueswhen the same number of photons are incident are different among thepixels, there is concern that photon counting accuracy is degraded whenbinning of the pixels is performed.

An object of an aspect of the present disclosure is to provide a photoncounting device and a photon counting method capable of curbingdegradation of photon counting accuracy.

Solution to Problem

A photon counting device according to an aspect includes a plurality ofpixels each including a photoelectric conversion element configured toconvert input light to charge, and an amplifier configured to amplifythe charge converted by the photoelectric conversion element and convertthe charge to a voltage; an A/D converter configured to convert thevoltage output from the amplifier of each of the plurality of pixels toa digital value and output the digital value; a correction unitconfigured to correct the digital value output from the A/D converter sothat an influence of a variation in a gain and an offset value among theplurality of pixels is curbed; a calculation unit configured to output asummed value obtained by summing the corrected digital values in atleast two pixels; and a conversion unit configured to convert the summedvalue output from the calculation unit to a number of photons.

In such a photon counting device, the voltage according to the photoninput to the photoelectric conversion element is output from theamplifier. The voltage is converted to the digital value by the A/Dconverter. When pixel binning is performed, the summed value obtained bysumming the digital values corrected by the correction unit is convertedto the number of photons. The correction unit corrects the digital valueso that an influence of the variation in the gain and the offset valueamong the plurality of pixels is curbed. That is, when the same numberof photons have been input, a variation in each pixel is curbed in thecorrected digital value. Thereby, it is difficult for the influence ofthe variation in the gain and offset value among the pixels to bereflected in the summed value, and it is easy for only the number ofphotons to be reflected in the summed value. Therefore, a degradation ofphoton counting accuracy can be curbed.

Further, the correction unit may have a parameter corresponding to thegain and the offset value, which is a preset parameter common to aplurality of pixels, and correct the digital value for each of theplurality of pixels on the basis of a deviation between the gain andoffset value and the parameter. In this configuration, since the digitalvalue is corrected according to a deviation between the parameterserving as a reference, and the gain and offset value, for example, thesummed value can be converted to the number of photons using a thresholdvalue based on the parameter.

Further, the readout noise of the amplifier may be equal to or less than0.2 [e-rms]. In this case, for example, an incorrect detection rate canbe curbed to 1% or less. Further, the readout noise of the amplifier maybe equal to or less than 0.15 [e-rms]. In this case, for example, theincorrect detection rate can be curbed to 0.1% or less.

Further, the gain may be equal to or more than 10 [DN/e]. By increasingthe gain, it is possible to accurately reproduce an analog value that isoutput from the amplifier.

Further, a photon counting method of an aspect includes converting lightinput to respective photoelectric conversion elements constituting aplurality of pixels to charge; amplifying, by an amplifier constitutingthe plurality of pixels, the converted charge and converting the chargeto a voltage; converting, by an A/D converter, the voltages output fromthe respective amplifiers to digital values and outputting the digitalvalues; correcting the digital value output from the A/D converter sothat an influence of a variation in a gain and an offset value among theplurality of pixels is curbed; summing the corrected digital valuescorresponding to at least two pixels and outputting a summed value; andconverting the summed value to the number of photons.

In such a photon counting method, the voltage output from the amplifieraccording to input photons is converted to the digital value. When pixelbinning is performed, the summed value obtained by summing the digitalvalues is converted to the number of photons. The digital value iscorrected so that the influence of the variation in the gain and theoffset value among the plurality of pixels is curbed. That is, when thesame number of photons have been input, a variation in each pixel iscurbed in the corrected digital value. Therefore, the influence of thevariation in the gain and the offset value among pixels is also curbedin the summed value. Therefore, a degradation of photon countingaccuracy can be curbed.

Further, the correcting of the digital value may include correcting thedigital value for each of the plurality of pixels on the basis of adeviation between the gain and offset value and a parameter, and theparameter may correspond to the gain and the offset value and may be setin advance to be common to the plurality of pixels. In thisconfiguration, since the digital value is corrected according to adeviation between the parameter serving as a reference, and the gain andoffset value, for example, the summed value can be converted to thenumber of photons using a threshold value based on the parameter.

Advantageous Effects of Invention

With the photon counting device and the photon counting method accordingto an aspect, it is possible to curb a degradation of photon countingaccuracy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a photon countingdevice according to an embodiment.

FIG. 2 is a graph showing a relationship between the number of electronsand a probability density.

FIG. 3 is a graph showing a relationship between readout noise and anincorrect detection rate.

FIG. 4(a) is a graph showing a relationship between the number ofelectrons and a probability density.

FIG. 4(b) is a graph showing a result of a simulation based on FIG.4(a).

FIG. 5(a) is a graph showing a relationship between the number ofelectrons and a probability density.

FIG. 5(b) is a graph in which a result of a simulation based on FIG.5(a) has been converted to a digital value.

FIG. 6(a) is a graph showing a relationship between the number ofelectrons and a probability density.

FIG. 6(b) is a graph in which a result of a simulation based on FIG.6(a) has been converted to a digital value.

FIG. 7(a) is a graph showing a relationship between the number ofelectrons and a probability density.

FIG. 7(b) is a graph in which a result of a simulation based on FIG.7(a) has been converted to a digital value.

FIG. 8 is a diagram schematically illustrating a process of converting ameasured digital value to a number of photons.

FIG. 9 is a diagram schematically illustrating a process of converting ameasured digital value to a number of photons.

FIG. 10 is a diagram schematically illustrating a process of deriving anoffset value.

FIG. 11 is a diagram schematically illustrating a process of deriving again.

FIG. 12 is a diagram illustrating a correspondence between a measureddigital value and a digital value after correction.

FIG. 13 is a flowchart illustrating an operation of the photon countingdevice.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments will be specifically described with referenceto the drawings. For convenience, substantially the same elements aredenoted by the same reference numerals, and a description thereof may beomitted. Further, photon counting in the embodiment includes bothcounting of the number of photoelectrons generated in each pixel of animage sensor and counting of the number of photons in consideration ofquantum efficiency (QE) of an image sensor.

First Embodiment

FIG. 1 is a diagram illustrating a configuration of a photon countingdevice. As illustrated in FIG. 1, the photon counting device 1 includesa CMOS image sensor 10, and a computer 20 connected to the CMOS imagesensor 10. The CMOS image sensor 10 includes a plurality of pixels 11and A/D converters 15. The plurality of pixels 11 are disposedtwo-dimensionally and arranged in a row direction and a columndirection. Each pixel 11 has a photodiode (a photoelectric conversionelement) 12 and an amplifier 13. The photodiode 12 accumulates electrons(photoelectrons) generated due to input of photons as charge. Theamplifier 13 converts the charge accumulated in the photodiode 12 to avoltage and amplifies the voltage. The amplified voltage is transferredto a vertical signal line 16 for each line (for each row) by switchingof a selection switch 14 of each pixel 11. A correlated double sampling(CDS) circuit 17 is disposed in each vertical signal line 16. The CDScircuit 17 removes noise that varies between pixels, and temporarilystores the transferred voltage.

The A/D converter 15 converts the voltage output from each of theamplifiers 13 in the plurality of pixels 11 to a digital value. In thefirst embodiment, the A/D converter 15 converts the voltage stored inthe CDS circuit 17 to a digital value. The respective digital valuesafter the conversion are output to the computer 20. For example, thedigital value may be sent to a horizontal signal line (not illustrated)by switching for column selection and output to the computer 20. Thus,in the CMOS image sensor 10, when photons are input to each pixel 11, adigital value according to the number of input photons is output to thecomputer 20. The A/D converter 15 may be provided in each pixel 11.

When the voltage amplified by the amplifier 13 is read, readout noise,which is random noise, is generated in the amplifier 13. FIG. 2 is agraph showing a probability distribution of electrons, in which ahorizontal axis represents the number of electrons and a vertical axisrepresents a probability density. The number of electrons generated bythe input photons follows a Poisson distribution, as illustrated in FIG.2. In FIG. 2, the probability distribution of electrons when two photonsare input to one pixel on average is shown for each readout noise.Examples of the readout noise include 0.12, 0.15, 0.25, 0.35, 0.40,0.45, and 1.0 [e-rms]. When the readout noise is smaller, a peak of awaveform of the probability distribution appears to be sharper andseparation of distributions for respective numbers of electrons becomesclear, as illustrated in FIG. 2. On the other hand, when the readoutnoise increases, distributions overlap with each other in the adjacentnumbers of electrons, and it becomes difficult to separate thedistributions of the respective numbers of electrons. For example, whenthe readout noise is equal to or less than 0.40 [e-rms], a peak for eachnumber of electrons appears identifiably. On the other hand, when thereadout noise is equal to or more than 0.45 [e-rms], it is difficult toidentify the peak for each number of electrons. In the first embodiment,a magnitude of the readout noise allowing distinguishment between thenumbers of electrons to be distinguished has been obtained according towhether or not the peak can be identified. Thereby, in the CMOS imagesensor 10 of the first embodiment, the readout noise is equal to or lessthan 0.4 [e-rms]. An inflection point may be detected by secondarilydifferentiating the probability distribution and the magnitude of thereadout noise allowing distinguishment between the numbers of electronsmay be obtained.

When a threshold value for distinguishing between the adjacent numbersof electrons has been set, an incorrect detection rate of the number ofdetected electrons changes according to readout noise. FIG. 3 is a graphshowing a relationship between the readout noise and the incorrectdetection rate when a threshold value is an intermediate value betweenthe numbers of electrons, such as 0.5e, 1.5e, 2.5e, . . . . Theincorrect detection rate is a rate at which an incorrect number ofelectrons is detected, and is caused by the spread of the probabilitydistribution of the electrons. As illustrated in FIG. 3, when theincorrect detection rate is desired to be equal to or less than 1%, thereadout noise needs to be equal to or less than 0.2 [e-rms]. Further,when the incorrect detection rate is desired to equal to or less than0.1%, the readout noise needs to be equal to or less than 0.15 [e-rms].

FIG. 4(a) is a graph showing a relationship between the number ofelectrons and the probability density. FIG. 4(b) is a graph showing aresult of a simulation based on FIG. 4(a). In FIG. 4(a), a probabilitydistribution of electrons when two photons are input to one pixel onaverage in a case in which readout noise is 0.15 [e-rms] is shown. InFIG. 4(b), a distribution of the number of electrons for each number ofmeasurements is shown by an analog value. The A/D converter 15 convertsthe analog value illustrated in FIG. 4(b) to a digital value and outputsthe digital value. The digital value output from each pixel 11 is shownby the following equation.Digital value[DN]=gain[DN/e]×number of electrons[e]+offset value[DN]

FIGS. 5(b), 6(b), and 7(b) show graphs when the analog value of FIG.4(b) has been converted to a digital value. In FIGS. 5(a), 6(a), and7(a), a probability distribution of electrons when two photons are inputto one pixel on average in a case in which readout noise is 0.15 [e-rms]is shown, as in FIG. 4(a). In FIGS. 5 to 7, threshold values fordistinguishing the numbers of electrons are set with reference tointermediate values between the numbers of electrons, such as 0.5e,1.5e, 2.5e . . . . In FIGS. 5 to 7, the threshold values are indicatedby broken lines. In FIG. 5(b), the gain is 2 [DN/e] and the offset valueis 100 [DN]. As illustrated in FIG. 5(b), when the gain is 2 [DN/e], itis difficult for a variation in a measured value observed in an analogvalue to be reflected on the graph. Further, a proportion of output ofdigital values showing the same value as the threshold value is high.

In FIG. 6(b), the gain is 10 [DN/e] and the offset value is 100 [DN]. Asillustrated in FIG. 6B, when the gain is 10 [DN/e], a distribution ofdigital values approximates to a distribution of analog values. On theother hand, since the gain is an even number, a digital valuecorresponding to the threshold value may be taken as illustrated in FIG.6(b). In FIG. 7(b), the gain is 11 [DN/e] and the offset value is 100[DN]. As illustrated in FIG. 7(b), when the gain is 11 [DN/e], thedistribution of the digital values approximates to the analog values.Further, since the gain is an odd number, taking a digital valuecorresponding to the threshold value is curbed. Thus, by increasing avalue of the gain, the output digital value can further approximate tothe analog value. In the first embodiment, the CMOS image sensor 10 mayhave, for example, a gain equal to or more than 10 [DN/e].

Refer back to FIG. 1. The computer 20 physically includes, for example,a storage device such as a RAM and a ROM, a processor (an arithmeticcircuit) such as a CPU, and a communication interface. Examples of sucha computer 20 include a personal computer, a cloud server, a smartdevice (a smartphone, a tablet terminal, or the like), a microcomputer,and a field-programmable gate array (FPGA). The computer 20 functions asa storage unit 21, a correction unit 22, a calculation unit 23, aconversion unit 24, a data processing unit 25, and a control unit 26,for example, by a CPU of a computer system executing a program stored ina storage device. The computer 20 may be disposed inside a cameraincluding the CMOS image sensor 10, or may be disposed outside thecamera. A display device 27 and an input device 28 can be connected tothe computer 20. The display device 27 is a display that can display,for example, a photon counting result obtained by the computer 20. Theinput device 28 is a keyboard, a mouse, and the like allowing a user toinput measurement conditions. A common touch screen may be used as thedisplay device 27 and the input device 28.

The storage unit 21 stores reference data for converting the digitalvalue output from the CMOS image sensor 10 to the number of photons. Thereference data includes, for example, a gain and an offset value foreach of the plurality of pixels 11. Further, the reference data includesthreshold value data for converting the digital value to a number ofphotons. The threshold value data may be prepared for a binning size.The binning size may be, for example, the number of pixels to be binned.In the case of 3×3 pixel binning, the binning size is “9”.

The correction unit 22 corrects the digital value corresponding to eachpixel output from the A/D converter 15. In the embodiment, the digitalvalue is corrected so that an influence of the variation in the gain andoffset value among the plurality of pixels 11 is curbed.

The calculation unit 23 outputs a summed value obtained by summing thedigital values after the correction of at least two pixels 11. In theplurality of pixels of which the digital values are summed, the numbersof pixels in a row direction may be the same as the number of pixels ina column direction, like 2×2 pixels, 3×3 pixels, or the like. Further,in the plurality of pixels of which the digital values are summed, thenumbers of pixels in the row direction may be different from the numberof pixels in the column direction, like 1×2 pixels, 2×5 pixels, or thelike. Further, the digital values after correction of all the pixelsconstituting the CMOS image sensor 10 may be summed.

The conversion unit 24 converts the summed value output from thecalculation unit 23 to the number of photons by referring to thethreshold value data stored in the storage unit 21. The data processingunit 25 creates a two-dimensional image indicating the number of photonsin each pixel 11 on the basis of the number of photons output from theconversion unit 24. Further, the data processing unit 25 may create, forexample, a histogram, which is a plot of the number of pixels withrespect to the number of photons. The created two-dimensional image orthe like can be output to the display device 27. The control unit 26 cangenerally control each function of the computer 20 or the CMOS imagesensor 10. For example, the control unit 26 controls the photon countingdevice 1 on the basis of setting conditions input by the input device28.

Next, details of the photon counting device 1 will be described whileshowing specific examples focusing on processes of the correction unit22, the calculation unit 23, and the conversion unit 24. Hereinafter, anexample in which the CMOS image sensor 10 of the photon counting deviceincludes a plurality of (nine) pixels 11 arranged in 3 rows×3 columns,and binning of 3×3 pixels is performed for simplicity of descriptionwill be described. The number of pixels in the row direction and thecolumn direction in binning can be designated by measurement conditionsinput to the input device 28.

First, a method of converting a digital value to a number of photonswhen it is assumed that there is no variation in the gain and the offsetvalue will be described. FIG. 8 schematically illustrates a process ofconverting the measured digital value to the number of electrons. In theexample of FIG. 8, it is assumed that the offset value is 100 [DN] andthe gain is 11 [DN/e] in each pixel. Further, the readout noise isassumed to be 0.15 [e-rms].

As illustrated in FIG. 8, in such a CMOS image sensor 10, when photonsare input to each pixel 11, charge is accumulated in each pixel 11according to the number of photons. In the illustrated example, fiveelectrons are accumulated in all the pixels 11. That is, 45 electronsare accumulated in the nine pixels. The accumulated charge is convertedto a voltage by the amplifier 13 and is converted to a digital value bythe A/D converter 15. In FIG. 8, the digital value in each pixel isshown inside the pixel.

The digital values in the respective pixels are summed in pixels thatare binning targets. In the example of FIG. 8, digital values (155, 153,155, 156, 154, 156, 156, 157, and 153) of nine pixels arranged in 3rows×3 columns are summed. Thereby, as illustrated in FIG. 8, a summedvalue of the digital values in the binned pixel 31 becomes 1395.

The summed value is converted to the number of electrons. In this case,the summed value is converted to the number of electrons using, forexample, a threshold value range. When an upper limit and a lower limitof the threshold value range are intermediate values of the number ofelectrons, a threshold value indicating the lower limit of each numberof electrons and a threshold value indicating the upper limit thereofare expressed by the following equations, respectively, and a range fromthe threshold value of the lower limit to the threshold value of theupper limit is a threshold value range corresponding to the number ofelectrons.Threshold value(lower limit)=(number of electrons−0.5)×gain+offsetvalue×binning sizeThreshold value(upper limit)=(number of electrons+0.5)×gain+offsetvalue×binning size

In the example of FIG. 8, it is assumed that the gain and the offsetvalue are 11 [DN/e] and 100 [DN], respectively, and there is novariation in the gain and the offset value between pixels, as describedabove. Therefore, for example, the lower limit of the threshold valuerange corresponding to 45 electrons is 1390 [DN], and the upper limitthereof is 1400 [DN]. When the digital value illustrated in FIG. 8 hasbeen converted to the number of electrons by referring to this thresholdvalue range, 1395 [DN], which is the summed value of the binned pixel31, is converted to 45 electrons. Since the number of electronsgenerated by the input photons follows a Poisson distribution, it ispossible to obtain the number of photons by dividing the convertednumber of electrons by a quantum efficiency. For example, when thequantum efficiency is 100%, the number of electrons and the number ofphotons are the same.

Next, a case in which a digital value is converted to the number ofelectrons using the same threshold value range as in FIG. 8 in a statein which the gain and the offset value have a variation will beconsidered. FIG. 9 illustrates an example of digital values when thegain and the offset value have a variation. In this example, an averagegain is 11 [DN/e], and a variation σ in the gain is 10%. That is, a gain±σ can have a value of 9.9 to 12.1. Further, an average offset value is100 [DN], and a variation σ in the offset value is 3%. That is, anoffset value ±σ can have a value from 97 to 103. In the example in FIG.9, a model in which five electrons are accumulated in all the pixels, asin FIG. 8, is shown. When digital values of nine pixels arranged in 3rows×3 columns are summed as in the example of FIG. 8, the summed valueof the binned pixels 31 becomes 1435 [DN]. When the threshold valuerange is obtained on the basis of the average offset value and theaverage gain as in the example of FIG. 8, the threshold value rangeincluding 1435 [DN], which is the summed value, corresponds to 49electrons. That is, 1435 [DN] is converted to 49 electrons. Thus, it maybe difficult to convert the digital value to a correct number ofelectrons in a state in which the gain and the offset value have avariation.

Therefore, in the photon counting device 1 of the embodiment, thecorrection unit 22 corrects the digital value output from the A/Dconverter 15 so that an influence of a variation in the gain and theoffset value among the plurality of pixels 11 is curbed. In theembodiment, since the correction unit 22 corrects the digital value ofeach pixel 11, an apparent gain and an apparent offset value are thesame in each pixel 11. The digital value after the correction can bederived on the basis of a deviation between the gain and offset value ofeach pixel 11 and the apparent gain and offset value common to all thepixels. For example, the digital value after the correction is derivedusing the following correction equation. The apparent gain and theapparent offset value (parameter) are set in advance and stored in thestorage unit 21.Digital value after correction=((digital value−offsetvalue)/gain)×apparent gain+apparent offset value

The offset value and the gain in the above correction equation areincluded in the reference data stored in the storage unit 21. Here, aprocess of acquiring the gain and the offset value will be described.FIG. 10 is a schematic diagram illustrating a process of acquiring anoffset value. The digital value is expressed by the following equationas described above. Therefore, the offset value is indicated as adigital value that is output from the CMOS image sensor 10 in a state inwhich no light is input. Therefore, when the offset value is acquired, adigital value output for each of a plurality of pixels is first obtainedon the basis of a plurality of dark images acquired by the CMOS imagesensor 10 in a state in which no light is input. The offset value isacquired by averaging the acquired digital values for each pixel.Digital value [DN]=gain [DN/e]×number of electrons [e]+offset value [DN]

FIG. 11 is a schematic diagram illustrating a process of acquiring again. When a gain of each pixel is acquired, a plurality of frame imagesare acquired by the CMOS image sensor 10 under a condition in which asufficient amount of light is given. An average optical signal value S[DN] of the digital value in each pixel and a standard deviation N [DN]are acquired. Since the gain is expressed by N²/S, the gain is derivedfrom the average optical signal value S and the standard deviation N.

FIG. 12 is a diagram illustrating a correspondence between the measureddigital value and a digital value after correction. In the example ofFIG. 12, each pixel 11 configuring the CMOS image sensor 10 has the gainillustrated in FIG. 10 and the offset value illustrated in FIG. 11. InThe example of FIG. 12, an example in which the digital value measuredby the CMOS image sensor 10 in FIG. 9 has been corrected by the abovecorrection equation is illustrated. In this example, the conversion unit24 corrects the digital value so that the apparent gain in all thepixels is 11 [DN/e] and the apparent offset value is 100 [DN]. That is,the digital value after the correction is derived using the followingcorrection equation.Digital value after correction=((digital value−offsetvalue)/gain)×11+100

The conversion unit 24 acquires the number of electrons using thresholdvalue data common to the respective pixels with respect to the digitalvalue after the correction. For example, the storage unit 21 may hold athreshold value range derived by the following equation as a table. Theconversion unit 24 can convert the digital value after the correction tothe number of electrons by referring to the threshold value data held inthe table. In the example of FIG. 12, the apparent gain is 11 [DN/e],and the apparent offset value is 100 [DN]. Therefore, in the binning of3×3 pixels, when the binning size is “9” and the digital value aftercorrection is 1390 to 1400, it is determined that the number ofelectrons is 45. The conversion unit 24 can obtain the number of photonsby dividing the converted number of electrons by the quantum efficiency.Threshold value(lower limit)=(number of electrons−0.5)×apparentgain+apparent offset value×binning sizeThreshold value(upper limit)=(number of electrons+0.5)×apparentgain+apparent offset value×binning size

Next, an operation of the photon counting device 1 will be described.FIG. 13 is a flowchart illustrating an operation of the photon countingdevice. In the embodiment, when the measurement is started in a state inwhich the photon counting device 1 is operated, light incident on thepixels of the CMOS image sensor 10 is first converted to charge by thephotodiode 12 (step S1). The converted charge is converted to a voltageby the amplifier 13 (step S2). The voltage is converted to a digitalvalue by the A/D converter 15 and output to the computer 20 (step S3).The digital value is corrected for each pixel by the correction unit 22of the computer 20 (step S4). The corrected digital value is binned(step S5). That is, the corrected digital values corresponding to thepixels 11 constituting the binned pixel 31 are summed and a summed valueis output. The summed value, that is, the binned digital value iscompared with the threshold value data (step S6), and is converted tothe number of photons on the basis of a comparison result (step S7).Thereby, the number of input photons is measured for each binned pixel.A measurement result, for example, may be displayed on the displaydevice 27 as image data or the like, or may be output as a numericalvalue.

As described above, in the photon counting device 1, a voltage accordingto the input photons is output from the amplifier 13. The voltage isconverted to a digital value by the A/D converter 15. When the binningof the pixel 11 is performed, a summed value obtained by summing thedigital values corrected by the correction unit 22 is converted to thenumber of photons. The correction unit 22 corrects the digital value sothat an influence of the variation in the gain and the offset valueamong the plurality of pixels 11 is curbed. That is, when the samenumber of photons are input, the variation for each pixel 11 is curbedin the corrected digital value. Thereby, it is difficult for aninfluence of the variation in the gain and offset value among the pixelsto be reflected in the summed value, and it is easy for only the numberof photons to be reflected in the summed value. Therefore, a degradationof photon counting accuracy can be curbed.

The correction unit 22 may have a parameter corresponding to the gainand the offset value, which is a preset parameter common to a pluralityof pixels, and correct the digital value for each of the plurality ofpixels on the basis of a deviation between the gain and offset value andthe parameter. In this configuration, since the digital value iscorrected according to the deviation between the parameter serving as areference, and the gain and offset value, for example, the summed valuecan be converted to the number of photons using a threshold value basedon the parameter.

The readout noise of the amplifier 13 may be equal to or less than 0.2[e-rms]. In this case, for example, the incorrect detection rate can becurbed to 1% or less. Further, the readout noise of the amplifier 13 maybe equal to or less than 0.15 [e-rms]. In this case, for example, theincorrect detection rate can be curbed to 0.1% or less.

The gain may be equal to or more than 10 [DN/e]. Since the CMOS imagesensor 10 has a high gain, it is possible to accurately reproduce ananalog value output from the amplifier 13 as a digital value.

Although the embodiments have been described in detail with reference tothe drawings, a specific configuration is not limited to theembodiments.

For example, in the CMOS image sensor 10 of the embodiment, an examplein which the readout noise of each pixel is equal to or less than 0.4[e-rms] has been described. However, even when the readout noise isequal to or less than 0.4 [e-rms] in a sensor specification, noise ofsome pixels may be greater than 0.4 [e-rms]. In such a case, pixels ofwhich the readout noise is equal to or less than 0.4 [e-rms] may beascertained in advance by measurement or the like, and photon countingmay be executed using only the pixels of which the readout noise isequal to or less than 0.4 [e-rms].

Further, an example in which the digital value after correction isobtained by the following equation has been illustrated, but the presentinvention is not limited thereto.Digital value after correction=((digital value−offsetvalue)/gain)×apparent gain+apparent offset value

For example, the digital value after the correction may be obtained bythe following equation.Digital value after correction=((digital value−offsetvalue)/gain)×apparent gain

In this case, for example, the storage unit 21 may hold a thresholdvalue range derived by the following equation as a table. The conversionunit 24 can convert the digital value after correction to the number ofelectrons by referring to the threshold value data held in the table.Threshold value(lower limit)=(number of electrons−0.5)×apparent gainThreshold value(upper limit)=(number of electrons+0.5)×apparent gain

Further, the digital value after the correction may be obtained by thefollowing equation.Digital value after correction=((digital value−offset value)/gain)

In this case, for example, the storage unit 21 may hold a thresholdvalue range derived by the following equation as a table. The conversionunit 24 can convert the digital value after the correction to the numberof electrons by referring to the threshold value data held in the table.Threshold value(lower limit)=(number of electrons−0.5)Threshold value(upper limit)=(number of electrons+0.5)

REFERENCE SIGNS LIST

-   -   1: photon counting device    -   11: Pixel    -   12: Photodiode (photoelectric conversion element)    -   13: Amplifier    -   15: A/D converter    -   21: Storage unit    -   22: Correction unit    -   23: Calculation unit    -   24: Conversion unit

The invention claimed is:
 1. A device for photon counting, the device comprising: a plurality of pixels each including a photoelectric conversion element configured to convert input light to charge, and an amplifier configured to amplify the charge converted by the photoelectric conversion element and convert the charge to a voltage; an A/D converter configured to convert the voltage output from the amplifier of each of the plurality of pixels to a digital value and output the digital value; and a computer configured to correct the digital value output from the A/D converter so that an influence of a variation in a gain and an offset value among the plurality of pixels is curbed, output a summed value obtained by summing the corrected digital values in at least two pixels, wherein the computer is configured to correct the digital value for each of the plurality of pixels based on a deviation between the gain and offset value and a parameter by referring to the parameter corresponding to the gain and the offset value, the parameter being a preset parameter common to the plurality of pixels, and convert the summed value to a number of photons.
 2. The device according to claim 1, wherein a readout noise of the amplifier is equal to or less than 0.2 [e-rms].
 3. The device according to claim 1, wherein a readout noise of the amplifier is equal to or less than 0.15 [e-rms].
 4. The device according to claim 1, wherein the gain is equal to or more than 10 [DN/e].
 5. A method for photon counting, the method comprising: converting light input to respective photoelectric conversion elements constituting a plurality of pixels to charge; amplifying, by an amplifier constituting the plurality of pixels, the converted charge and converting the charge to a voltage; converting, by an A/D converter, the voltage output from the amplifier to digital value and outputting the digital value; correcting the digital value output from the A/D converter so that an influence of a variation in a gain and an offset value among the plurality of pixels is curbed, wherein the correcting the digital value includes correcting the digital value for each of the plurality of pixels based on a deviation between the gain and offset value and a parameter, and the parameter corresponds to the gain and the offset value and is set in advance to be common to the plurality of pixels; and summing the corrected digital values corresponding to at least two pixels and outputting a summed value; and converting the summed value to a number of photons.
 6. The method according to claim 5, wherein readout noise of the amplifier is equal to or less than 0.2 [e-rms].
 7. The method according to claim 5, wherein readout noise of the amplifier is equal to or less than 0.15 [e-rms].
 8. The method according to claim 5, wherein the gain is equal to or more than 10 [DN/e]. 